The mechanisms underlying prostate cancer progression from androgen dependent to castration resistant prostate cancer (CRPC), as well as those mediating resistance to therapies that target the residual androgen receptor (AR) signaling in CRPC including second generation AR antagonists such as enzalutamide have not been fully elucidated. Work from several groups including our own has highlighted the continued dependence of CRPC on AR and has implicated EZH2 as an additional oncogenic driver involved in AR reprogramming in CRPC. In studies accomplished during the current funding period we have explored EZH2 as a target in models of CRPC and have demonstrated that catalytic EZH2 inhibitors have activity in these models. The goal of this proposal is to leverage the novel genome-scale CRISPR/Cas9 and ORF screening technology to identify the essential genes and their functions that underlie the hormone independence and sensitivity or resistance to EZH2 inhibitors, AR antagonists, and other targeted agents in CRPC. In addition, in collaboration with the Freedman lab we will use CRISPR/Cas9 editing of cis-regulatory elements to explore in detail the function of a novel somatically acquired transcriptional enhancer in controlling the expression of the AR gene itself in CRPC. We will also define the essential genes in models of CRPC driven by ARv7 and other AR mutants and variants in collaboration with Project 2. Finally, we will perform CRISPR and ORF screens to identify potential new synthetic lethal combinations and mechanisms of resistance including to a set of novel kinase inhibitors in collaboration with the Project 3.